Set headphone volume level on Intel Macs
Macworld: Mac OS X Hints: Set headphone volume level on Intel Macs
If you’ve got a new Intel-powered Mac, here’s a feature you may not have even known you had. For years, all Macs have had the ability to have different volume levels for different inputs. Plug in a USB-powered iMic, for instance, and you can set its output volume level independently of that of your internal speakers.